The present invention relates to a compact antenna and, in particular, to a compact lightweight antenna configuration which is adaptable to mobile, hand-held communications devices, including transceivers and including those devices which operate at high frequencies (VLF is considered to about 60 Hz and HF is about 2-30 MHz).
The need for the present invention arises, for example, in response to the continuing development of LSI and VLSI circuits for receivers, transmitters and transceivers. As is frequently the case regarding integrated circuit development, the associated physical hardware, in this case antennas, has not kept pace with the miniaturization of the integrated circuit components. The need which led to the development of the present invention derived from the requirements of a miniaturized hand-held communications terminal which is used in a computer-controlled restaurant or institutional ordering and billing system, or an inventory control system. This hand-held terminal incorporates the antenna of the present invention and is referenced here to illustrate, without limitation, the application and operation of that antenna. The hand-held terminal and the associated institutional/inventory control system are the subject of co-pending U.S. patent application, Ser. No. 655,019, entitled AUTOMATED ORDERING AND ACCOUNTING SYSTEM, filed in the names of Charles P. Thcaker, Frederick J. Scholz and Robert T. Bryant, on the same date as the present application, which application is assigned to the assignee of the present application and is incorporated by reference.
In the recent past, frequent attempts have been made to derive from the dipole antenna small antennas which are suitable for small integrated circuit radio devices (transmitters, receivers, transceivers). Consider first the half-wave dipole antenna 30 shown in FIG. 2. In free space, such an antenna is one piece of wire stretched out the full half wavelength of the frequency of operation. The antenna is fed by a transmission line 31, usually via an insulator 32 inserted in the middle of the wire so that each end of the dipole 33--33 is one-quarter wavelength from the center.
There are a number of approaches for shortening such a dipole antenna by inductive loading. A typical approach involves inserting coils 34--34 in the one-quarter wavelength wire. Inserting an inductance, however, introduces reactance, making it difficult to obtain a good match at the resonating frequency and, thus, requiring compensation for the reactance. It also narrows the bandwidth of the antenna. If the introduced inductance is very small compared to the length of the dipole, the inductive reactance can generally be ignored. However, if the antenna comprises a large percentage of coil and a small percentage of wire, or comprises essentially all coil, then it is necessary to compensate for that inductive reactance, and the bandwidth becomes extremely narrow. One approach is to broaden the dipole ends with plates or wires 35, FIG. 2, to provide a capacitive coupling into space at the wire ends.
Referring to FIG. 3, there is illustrated schematically a second type of dipole antenna, a conventional quarterwave whip antenna 40 which is fed against ground. The coaxial cable transmission line 41 is connected so that its shielded outer conductor is coupled to ground and the internal conductor from the radio or other transmission/receiving device is connected to one end of the straight, one-quarter wavelength antenna wire 42. Such antenna systems are usually mounted on a metal surface or on the ground. The radiated electromagnetic waves are then reflected off the ground or surface and so develop an overall symmetrical pattern of radiation in all directions at a rather low angle of radiation. More importantly, the length of the antenna, 1.sub.a, is inversely proportional to frequency, f, and directly proportional to wavelength, .lambda.. That is, 1.sub.a .varies..lambda..varies.1/f. As a consequence, at low frequencies 1.sub.a is very long. As is true of the half-wave dipole antenna, the quarter-wave whip antenna can be shortened by inserting an inductor such as a coil 43, in this case at the base. Again, however, the insertion of inductance into the antenna changes the reactance and narrows the bandwidth and at some point it becomes necessary to compensate, for example, by attaching "capacity hats" to the antenna.
The patent literature reflects several approaches which have been utilized in attempting to provide small, lightweight antennas. Illustrative of one of the several approaches, Hooper, U.S. Pat. No. 3,049,711, discloses an omni-directional antenna comprising two tuned coil circuits. The first coil is included in a first tuned circuit with a first capacitance. The second coil is formed on a printed circuit along with a second capacitance and forms the second tuned circuit. The two circuits are resonant at the same frequency.
In relevant part, the Hooper '711 patent is of general interest in teaching (1) the use of a planar printed circuit coil in a tuned oscillator circuit and (2) the use of a printed circuit dielectric board or paper to define a capacitor which is coupled to a planar printed circuit coil.
A second approach for miniaturizing antennas, believed applicable to VHF systems, involves the use of loop antennas. For example, Rennels et al, U.S. Pat. No. 3,736,591, discloses a U-shaped pager antenna which is formed by the walls of the pager housing and functions as an inductive loop antenna to detect the H-field associated with the transmitted electromagnetic signal. Nagata et al, U.S. Pat. No. 4,123,756, also discloses a U-shaped looped miniature radio antenna. In this implementation, the antenna is formed by a conductive lining or a plated film which is formed inside the two major walls and the adjoining end wall of the radio housing. James, Jr. et al, U.S. Pat. No. 3,956,701, discloses a printed circuit antenna construction for a pager in which two, three-dimension, selectively tuned/detuned orthogonal antennas are formed by planar conductor arrays on the opposite sides of a folded printed circuit board.
Still another approach is encompassed in the transceiver dual-mode antenna of Garay et al, U.S. Pat. No. 4,313,119. Garay et al provides a collapsible or foldable whip-type dipole antenna (or a folding meander line dipole antenna) and a U-shaped loop antenna which is formed on the transceiver casing. Extension or unfolding of the dipole antenna element decouples the loop antenna. Upon retracting or folding, the dipole antenna merges with the loop antenna and couples the loop antenna to the transceiver.
None of the above-described antennas and, to our knowledge, none of the existing prior art antennas provide the combination of small size and weight and the ready adaptability to a range of frequencies, including high frequencies, which are necessary to applications such as the communications terminal described herein.